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[2003:cb:c713:3200:77d:8652:169f:b5f7]) by smtp.gmail.com with ESMTPSA id cl5-20020a5d5f05000000b0033d1b760125sm5351759wrb.92.2024.03.01.09.19.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 01 Mar 2024 09:19:00 -0800 (PST) Message-ID: <861321e8-6b96-4cbf-a61b-d9cbc2dc093e@redhat.com> Date: Fri, 1 Mar 2024 18:18:59 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags To: Ryan Roberts , Matthew Wilcox Cc: Andrew Morton , Huang Ying , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <6541e29b-f25a-48b8-a553-fd8febe85e5a@redhat.com> <2934125a-f2e2-417c-a9f9-3cb1e074a44f@redhat.com> <049818ca-e656-44e4-b336-934992c16028@arm.com> <4a73b16e-9317-477a-ac23-8033004b0637@arm.com> <1195531c-d985-47e2-b7a2-8895fbb49129@redhat.com> <5ebac77a-5c61-481f-8ac1-03bc4f4e2b1d@arm.com> <7774edad-a194-4259-a95f-88bcef846f90@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 3855D1C0016 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: q6334u4fb73ttbcjpjkkmm9q518tbsdb X-HE-Tag: 1709313545-260123 X-HE-Meta: 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 YfamincT dj2EG/nGUiOijFMP2CJHYPDF63PyN2rba54tIwh7DFgb9Ocmn0WTsu7ypL8xi9JLFh8yoYd7AZokYn1zH/VSpHNUy+kuQokbybFv5uTFtVinDs+/vr1ETl1Y4kWCCOLUQDqY/8ak+2c+j3MXFXCKuDzIGXMcTGoKkBNmN6kGBdbLOoVFSYufEJS09R4F/HX5tl0WsjFGJW2I/sVQxHS9jhS1C8Q== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 01.03.24 18:14, Ryan Roberts wrote: > On 01/03/2024 17:00, David Hildenbrand wrote: >> On 01.03.24 17:44, Ryan Roberts wrote: >>> On 01/03/2024 16:31, Matthew Wilcox wrote: >>>> On Fri, Mar 01, 2024 at 04:27:32PM +0000, Ryan Roberts wrote: >>>>> I've implemented the batching as David suggested, and I'm pretty confident it's >>>>> correct. The only problem is that during testing I can't provoke the code to >>>>> take the path. I've been pouring through the code but struggling to figure out >>>>> under what situation you would expect the swap entry passed to >>>>> free_swap_and_cache() to still have a cached folio? Does anyone have any idea? >>>>> >>>>> This is the original (unbatched) function, after my change, which caused >>>>> David's >>>>> concern that we would end up calling __try_to_reclaim_swap() far too much: >>>>> >>>>> int free_swap_and_cache(swp_entry_t entry) >>>>> { >>>>>     struct swap_info_struct *p; >>>>>     unsigned char count; >>>>> >>>>>     if (non_swap_entry(entry)) >>>>>         return 1; >>>>> >>>>>     p = _swap_info_get(entry); >>>>>     if (p) { >>>>>         count = __swap_entry_free(p, entry); >>>>>         if (count == SWAP_HAS_CACHE) >>>>>             __try_to_reclaim_swap(p, swp_offset(entry), >>>>>                           TTRS_UNMAPPED | TTRS_FULL); >>>>>     } >>>>>     return p != NULL; >>>>> } >>>>> >>>>> The trouble is, whenever its called, count is always 0, so >>>>> __try_to_reclaim_swap() never gets called. >>>>> >>>>> My test case is allocating 1G anon memory, then doing madvise(MADV_PAGEOUT) >>>>> over >>>>> it. Then doing either a munmap() or madvise(MADV_FREE), both of which cause >>>>> this >>>>> function to be called for every PTE, but count is always 0 after >>>>> __swap_entry_free() so __try_to_reclaim_swap() is never called. I've tried for >>>>> order-0 as well as PTE- and PMD-mapped 2M THP. >>>> >>>> I think you have to page it back in again, then it will have an entry in >>>> the swap cache.  Maybe.  I know little about anon memory ;-) >>> >>> Ahh, I was under the impression that the original folio is put into the swap >>> cache at swap out, then (I guess) its removed once the IO is complete? I'm sure >>> I'm miles out... what exactly is the lifecycle of a folio going through swap out? >> >> I thought with most (disk) backends you will add it to the swapcache and leave >> it there until there is actual memory pressure. Only then, under memory >> pressure, you'd actually reclaim the folio. > > OK, my problem is that I'm using a VM, whose disk shows up as rotating media, so > the swap subsystem refuses to swap out THPs to that and they get split. To solve > that, (and to speed up testing) I moved to the block ram disk, which convinces > swap to swap-out THPs. But that causes the folios to be removed from the swap > cache (I assumed because its syncrhonous, but maybe there is a flag somewhere to > affect that behavior?) And I can't convince QEMU to emulate an SSD to the guest > under macos. Perhaps the easiest thing is to hack it to ignore the rotating > media flag. I'm trying to remember how I triggered it in the past, I thought cow.c selftest was able to do that. What certainly works is taking a reference on the page using vmsplice() and then doing the MADV_PAGEOUT. But there has to be a better way :) I'll dig on Monday! -- Cheers, David / dhildenb